1. Field of the Invention
The present invention relates to a vehicle body frame for a saddle-type vehicle such as a motorcycle, and particularly to a vehicle body frame having an improved structure of a head pipe provided at a front end portion of the vehicle body frame. The present invention also relates to a saddle-type vehicle incorporating the described vehicle body frame.
2. Description of the Background Art
A conventional vehicle body frame for a motorcycle is known, which includes a head pipe which pivotally supports a steering shaft at a front end portion thereof. Main frame attachment portions are provided at left and right upper portions of a rear portion of the head pipe, and front end portions of main frames provided in a left and right pair are welded to these attachment portions. A down frame attachment portion is provided integrally in a downwardly neighboring relationship with each of the main frame attachment portions, and a front end portion of a down frame is welded to the down frame attachment portion. Also it is known that the head pipe is formed by forging.
Such a known type of vehicle body frame is disclosed, for example, in published Japanese Patent Application No. 2008-230416.
Since high strength and low weight are required for the head pipe which is a portion for suspending a front wheel suspension, the head pipe is sometimes forged from an aluminum alloy or the like. In this case, since the main frame attachment portion is a high-load portion to which a higher load is applied, than that applied to the front wheel, high strength is required. High strength is also required for the down frame attachment portion neighboring with the main frame attachment portion.
Therefore, in the conventional vehicle frame, after the head pipe is forged, an insertion portion having a recessed groove shape is formed by mechanical working of the down frame attachment portion, and a front portion of the down frame is fitted into the insertion portion to secure a long fitting length. Then, the fitted portion is welded to form a long welding distance.
However, if the insertion portion having the long fitting length is formed in this manner, then the mechanical working becomes complicated and the number of working steps increases, and also the working time increases. Therefore, the cost of mechanical working increases. Further, the cost for the welding increases as the welding distance increases. Further, in order to integrate the main frame attachment portion and the down frame attachment portion with each other, it is necessary to increase the size of the head pipe. However, since the material waste which is removed by the mechanical working increases, the amount of required material increases, which increases the material cost. Accordingly, a structure which does not cause such increase of the cost is demanded.
Further, since the down frame attachment portion neighbors with the main frame attachment portion which is a high-load portion, the welding quality of the down frame attachment portion has a significant influence on the strength. Especially, if a double bead at which a welding portion of the main frame attachment portion and a welding portion of the down frame attachment portion overlap with each other occurs, then the stability of the welding quality is lost.
Therefore, a need exists for an improved head pipe structure which does not depend upon the welding quality.